Fluid-energy mills are used for reducing particle size of a variety of materials such as pigments, agricultural chemicals, carbon black, ceramics, minerals and metals, pharmaceuticals, cosmetics, precious metals, propellants, resins, toner and titanium dioxide. The particle size reduction typically occurs as a result of particle-to-particle collisions and particle collision with the walls.
Most fluid-energy mills are variations on a basic configuration of a disc-shaped grinding chamber enclosed by two, generally parallel, circular plates defining axial walls, and an annular rim defining a peripheral wall, with the axial length or height of the chamber being substantially less than the diameter. Around the circumference of the mill are located a number of uniformly spaced jets for injecting the grinding fluid which furnishes additional energy for comminution, along with one or more feed nozzles for feeding the particulate material to be comminuted. The jets are oriented such that the grinding fluid and particulate material are injected tangentially to the circumference of a circle smaller than the chamber circumference. Feed to the grinding chamber can be introduced either through a side inlet that is tangent to the grinding chamber, or at an angle from the top, usually at a 30° angle to the plane of the grinding chamber. Side feed micronizers generally produce the better grinding dispersion, while top feed micronizers can produce higher rates.
Within the grinding chamber, a vortex is formed by the introduction of the grinding fluid such as compressed gas, through the feed inlet or through fluid nozzles positioned in an annular configuration around the periphery of the grinding chamber. The grinding fluid (compressed gas, e.g., air, steam, nitrogen, etc.), fed tangentially into the periphery of the chamber, forms a high-speed vortex as it travels within the grinding chamber. The high-speed vortex sweeps up the particulate material, which results in high speed particle-to-particle collisions as well as collisions with the interior portion of the grinding chamber walls.
The grinding fluid velocity can be resolved into a tangential component of the velocity, Vt, which is a measure of the centrifugal force acting on the particle tending to keep it at the outer periphery of the chamber, and the radial component of velocity, Vr, which is a measure of the drag force generated by the action of the fluid against the particle tending to force the particle towards the central discharge conduit. By proper selection of conditions, such as rate and tangency of fluid injection, these opposing forces can be adjusted such that particles above a specific size are retained within the mill until sufficient attrition occurs, both by collision with other particles and the chamber walls, to reduce them to the desired sizes, up to the point when the drag forces become dominant over centrifugal forces and the particles are swept into the central discharge conduit that is coaxial to, and in direct communication with, the grinding chamber, and subsequently into a cyclone or bag filter for collection.
Clearly, heavier particles have longer residence time within the vortex. Lighter particles (i.e., those sufficiently reduced particles) move with the vortex of gas until the discharge conduit is reached. Typically, fluid-energy mills are capable of producing fine (less than 10 microns) and ultra fine (less than 5 microns) particles.
During grinding, undesirably large particle sizes frequently escape into the product. In other words, better classification of particulate material is required. Also during grinding, the inside liner of the grinding chamber is subjected to abrasion. Generally, the liner is a large casting of silicon carbide and because of its size has many joints. The joints are a potential source of problem. Particulate material and grinding fluid such as steam circulate around the liner at a velocity, for example, of about Mach 1. The momentum of even small particles at such velocities is very high. The liner is susceptible to abrasion and penetration by such particles, particularly where a joint occurs in the ceramic liner of the grinding chamber.
The fluid-energy mill of the present invention overcomes these problems, in that, it helps retain larger particles for a longer time in the chamber and discharging them only after required attrition is achieved, and it provides for interlocking joints that prevent penetration of particulate material into the ceramic liner. Consequently, the inside liner of the fluid-energy mill of the present invention lasts two to three times longer than a standard liner. Secondly, it produces a much narrower particle size distribution and a smaller number of large particles, thus improving quality. Finally, it can run at about 20% higher rate with a lower motive gas requirement in the grinding chamber.